pKa acetylene ~ 25

pKA ethylene ~ 44

I'm not looking for the "because alkyenes are sp hybridized and more s character means their electrons are held tighter" answer because I'm not the biggest fan of hybrid atomic orbital theory.

Is there an alternative answer that looks at this question from a Molecular Orbital (MO) Theory perspective instead? Or maybe a different conceptual analysis as to why the electron pair is more stable on the alkyene carbon than an alkene carbon? Does magnetism (from the ring of charge in the triple bond) somehow have an effect?

Or is it really as simple as the fact that the because the pi electrons are higher energy and farther away from the nucleus, the remaining electrons participating in sigma bonding must counteract the strong Coulombic pull from the carbon nucleus by moving closer to it?

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    $\begingroup$ hybrid orbital explanation: chemistry.stackexchange.com/questions/32341/… $\endgroup$ Commented Oct 24, 2016 at 18:06
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    $\begingroup$ Three of the carbons electron pairs hang on one side, so it is pretty positive on the other side, and pulls the CH electron pair closer. But blunt, that explanation, but what's wrong with it? Whatever you come up with, it is never more that a rationale. Nature does not answer "why" questions. ;-) $\endgroup$
    – Karl
    Commented Oct 25, 2016 at 0:10


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